Polar Ligand Adsorption Controls Semiconductor Surface Potentials

Controlled surface modification of CdTe single crystals and CdTe and CuInSe₂ solar cell quality thin films was achieved by chemisorption of a series of organic ligands with varying dipole moments. Contact potential difference measurements in air showed that adsorption of benzoic or hydroxamic acid derivatives on the thin films or crystals changes the semiconductors' electron affinity without significantly affecting band bending. The magnitude and direction of surface potential changes, which reach 670 mV between extreme modifications, correlate with the ligands' dipole moments. Ligand dipole moments were controlled by varying the substituents of the ligand. Quantitative Fourier transform infrared (FTIR) spectroscopy showed that benzoic acid surface coverage is about one monolayer. Finally, FTIR spectral analysis showed that the benzoic acid derivatives adsorb via coordination to Cd on CdTe and that hydroxamic acids bind to Cd on CdTe and to In on CuInSe₂. These phenomena occur in several systems (two semiconductor compounds, two types of binding groups, and two types of surface morphologies were examined) and may prove useful in band edge engineering.